Parakeets are flock creatures. They love to live in groups. That being said, YOU can of course be a part of the parakeet's "group". It all comes down to how much time you spend at home with your parakeet.
Let's say you're a stay at home elderly person who is in the living room 99% of wakeful hours. Your keet sleeps at night when you do, so it doesn't matter where you are when the keet is asleep :) If you are always in the room with the keet, the keet will be completely happy having you as a companion. There's no need for another bird.
However, if you are away at school all day long, and only get home at 6pm just before your keet is falling asleep, then your keet is going to be alone all day. That is really really cruel to do to a parakeet. Parakeets are meant to live in giant flocks, wheeling around in the sky. They consider being alone to be a very dangerous thing and in the wild they would do everything in their power to get back to the safety of the flock.
If you want to teach your keet to talk, then it is fine to have the keet alone for its first 6 months or so, while you work on that. Once that time is past, then it's really wise to get a second bird if you're not going to be around a lot.
Note that you should NEVER get 2 birds of opposite genders unless you have talked to a vet, gotten books on breeding and bought the equipment and medical supplies necessary. Breeding is a serious hobby to get into, and you need to be prepared beforehand.